Generating a pilot tone for an optical telecommunications system

ABSTRACT

The disclosure is directed to a method and system for generating a pilot tone for an optical signal with an optical telecommunications system. The pilot tone is generated in the digital domain by modulating the data to be transmitted to a destination node within the optical telecommunications network. The modulation of the data introduces occurrence modulation to the optical signal.

FIELD OF THE DISCLOSURE

The current disclosure is generally directed to opticaltelecommunication systems and, more specifically, is directed to amethod and system for generating a pilot tone for an opticaltelecommunications system.

BACKGROUND

The use of optical telecommunication systems, or networks, is growingand technology in this field is also improving. Opticaltelecommunication systems typically include a set of nodes whichcommunicate with each other. In Dense Wavelength Division Multiplexing(DWDM) systems, light at multiple wavelengths is modulated with streamsof digital information, and then the modulated light beams at differentwavelengths, termed “wavelength channels”, are combined for jointpropagation in an optical fiber.

To identify wavelength channels in a DWDM system, a pilot tone may beapplied to communication channels within the optical telecommunicationssystem. The pilot tone is typically a low frequency modulation of awavelength channel's optical power level. The pilot tone carriesinformation associated with the wavelength channel, such as, but notlimited to, its wavelength and other identification information forsupervisory, control, equalization, continuity, synchronization, orreference purposes.

By providing this pilot tone, communication between the transmittingnode and the receiving node over the communication channel is improvedas there is more information shared. The pilot tone also allows for datasharing between the different nodes within the opticaltelecommunications network. Detrimentally, the introduction of pilottones requires a dedicated optical modulator or variable opticalattenuator for each wavelength channel, which increases equipment costand complexity, especially for optical communication systems carryingmany wavelength channels.

Therefore, there is a need for an improved system and method foridentifying wavelength channels in optical telecommunication systems.

SUMMARY

The following presents a simplified summary of some aspects orembodiments of the disclosure in order to provide a basic understandingof the disclosure. This summary is not an extensive overview of thedisclosure. It is not intended to identify key or critical elements ofthe disclosure or to delineate the scope of the disclosure. Its solepurpose is to present some embodiments of the disclosure in a simplifiedform as a prelude to the more detailed description that is presentedlater.

The disclosure is directed to a method and system for generating a pilottone for an optical signal being transmitted in an opticaltelecommunications system. The pilot tone is generated in the digitaldomain, such as by a transmitter within the optical telecommunicationssystem. The generation of the pilot tone in the digital domain providesvarious advantages over current methods of pilot tone generation, whichtypically occurs in the analog domain.

In one embodiment, the method includes the generation of occurrencemodulation for the pilot tone. The system may include an apparatus forpre-conditioning data that is being transmitted from a transmitterwithin the telecommunications system and then modulates thispre-conditioned data before converting the digital signal to an analogcounterpart for delivering of the data (in the form of an opticalsignal) to a receiver at a destination node.

In another embodiment, a data driver supplies the information that is tobe transmitted from the transmitting node to the receiving, ordestination, node. A transmitter within the transmitting node mayinclude a pre-conditioning apparatus which performs various processes onthe information. This pre-conditioned information may be modulatedbefore being converted to an analog signal for transmission to thedestination node.

In one aspect of the disclosure, there is provided a method ofgenerating a pilot tone for an optical telecommunication system thatincludes providing digital data having a data rate, for transmission toa destination node and then modulating the digital data at a pilot tonefrequency lower than the data rate, to provide a modulated digitaloutput. The modulated digital output is then converted to an analogsignal to generate the pilot tone.

In another aspect, the modulation is performed to a peak-to-peakmodulation depth less than a least significant bit (LSB) of thedigital-to-analog conversion. In another aspect, the peak-to-peakmodulation depth is less than 10% of the LSB of the digital-to-analogconversion.

In yet a further aspect, before modulating the digital data, the digitaldata is pre-conditioned to add a pre-determined amount of randomness tothe digital data. In one aspect, the pre-conditioning is performed toadd a random or pseudo-random value of no greater than +/−0.5 of the LSBto each data point within the digital data.

In a further aspect, modulation of the digital data multiplying thedigital data by a modulation factor.

In yet another aspect, there is provided a system for generating a pilottone for an optical telecommunications link including a data driver forproviding digital data having a data rate; and a modulator formodulating the digital data provided by the data driver at a pilot tonefrequency lower than the data rate, to provide a modulated digitaloutput. The system further includes a digital-to-analog converter (DAC)for receiving the modulated digital output and converting the modulateddigital output to an analog signal to generate the pilot tone. In oneaspect, the modulator, which can be an apparatus such as a multiplier,is configured to modulate the digital data at a modulation depth lessthan a LSB of the DAC.

In another aspect of the disclosure, the system further includes apre-conditioning apparatus configured for pre-conditioning the digitaldata to add a pre-determined amount of randomness to the digital data.In one embodiment, the pre-conditioning apparatus is configured to add arandom or pseudo-random value of no greater than +/−0.5 of the LSB toeach data point within the digital data.

DESCRIPTION OF THE FIGURES

Embodiments of the present disclosure will now be described, by way ofexample only, with reference to the attached figures.

FIG. 1a is a schematic diagram of an optical telecommunications system;

FIG. 1b is a schematic diagram of a channel within the opticaltelecommunications system of FIG. 1 a;

FIG. 2 is an example of a pilot tone;

FIG. 3a is a schematic diagram of apparatus for generating occurrencemodulation for a pilot tone;

FIG. 3b is a flowchart outlining a method for generating occurrencemodulation for a pilot tone;

FIGS. 4a to 4c are simulation graphs of modulation depths for digital toanalog convertors;

FIGS. 5a to 5d are representations of optical signal intensities forvarious modulation depths for a 6-bit DAC;

FIGS. 6a to 6d are representations of optical signal intensities for aconstant modulation depths for different bit DACs;

FIGS. 7a and 7b are graphical representations of occurrence modulation;

FIGS. 8a and 8b are representations of spectral analysis of anoccurrence modulation signal;

FIG. 9 is a graphical representation of performance analysis of anoccurrence modulation signal; and

FIGS. 10a and 10b are graphical representations of experimentallyobtained data.

DETAILED DESCRIPTION

The following detailed description contains, for the purposes ofexplanation, numerous specific embodiments implementations, examples anddetails in order to provide a thorough understanding of the disclosure.It is apparent, however, that the embodiments, may be practiced withoutthese specific details or with an equivalent arrangement. In otherinstances, some well-known structures and devices are shown in blockdiagram form in order to avoid unnecessarily obscuring the embodimentsof the disclosure. The description should in no way be limited to theillustrative implementation, drawings and techniques illustrated below,including the exemplary designs and implementations illustrated anddescribed herein, but may be modified within the scope of the appendedclaims along with their full scope of equivalents.

The disclosure is directed to a method and system for generating a pilottone for an optical telecommunications system. In one embodiment, thepilot tone is generated in the digital domain with occurrencemodulation. By generating the pilot tone in the digital domain,advantages over some current solutions may be realized.

Currently, when a transmitter (within the transmitting node) delivers anoptical signal, the pilot tone is added to the analog optical signal viaa variable optical attenuator (VOA) which modulates the power level ofthe optical signal. The VOA level of attenuation is controlled by atime-varying control signal. Using this approach, it is generally hardto achieve high frequency modulation. In another current solution, thepilot tone (or modulation) is added through a data driver within thetransmitter, however this requires feedback control and calibration asthis entire process is being performed in the analog domain.

Turning to FIG. 1a , a schematic diagram of an opticaltelecommunications system is shown. The optical telecommunicationssystem, or optical network, 100 includes a set of nodes 102 a-102 gwhich are connected with each other via individual optical transmissionfibers 103. The nodes 102 may be connected with each other via more thanone transmission fiber 103. Signals transmitted along these individualtransmission fibers 103 produce a plurality of wavelength channels 104,each wavelength channel including light at a particular wavelengthmodulated with a high-speed digital stream of information. For ease ofunderstanding, a transmission node is seen as the node 102 a which isdelivering data while a destination, or receiving, node 102 g is seen asa node which is receiving the data. In some cases, communication betweentwo nodes may not be direct. By way of example, communication betweenthe transmission node 102 a and the destination node 102 g is such thatthe data passes through other nodes, such as 102 b and 102 e between thetransmission node 102 a and the destination node 102 g.

Within the network 100 are a set of pilot tone detectors 106 whichmonitor channel information, or characteristics, of the channels 104 bydetecting pilot tones which are modulated onto the wavelength channels104 traveling between the transmission nodes 102 a-102 g. Thesecharacteristics may include, but are not limited to, source/destinationidentification (ID), wavelength, power, modulation format or baud rateor any other characteristics or combination of those characteristics. Inone embodiment, the pilot tone detectors 106 include a low-speedphotodiode and a digital signal processor (DSP).

Turning to FIG. 1b , a schematic diagram showing a portion of theoptical telecommunications system of FIG. 1 a is provided. FIG. 1b showsthe connection between two nodes 102 a and 102 b which are connected bythe transmission fiber 103. Optical amplifiers 105 are provided toamplify the wavelength channels 104. In FIG. 1b , one of the nodes 102 amay be seen as the source, or transmission, node, and the second of thenodes 102 b may be seen as the destination node. In the embodiment ofFIG. 1b , individual pilot tone detectors 106 are connected at thebeginning and the end of the transmission fiber 103 between the twonodes 102, however, the pilot tone detectors 106 may be located anywherewithin the communication channel. Depending on the type of nodes 102 aor 102 b, the nodes 102 a or 102 b may include a reconfigurable opticaladd/drop multiplexor (ROADM) 107 coupled to a receiver 110 and atransmitter 112 for dropping and adding wavelength channels. Forexample, if the node 102 is an amplifier node, the node 102 would notinclude any receiver or transmitter, however, if the node 102 is anaccess node, there may be 0, one or multiple receivers 110 andtransmitters 112. Accordingly, the wavelength channels 104 may terminateat a destination node 102 b or propagate further, as shown with an arrowto the right of the node 102 b.

Turning to FIG. 2, a schematic time trace of a pilot tone modulatedwavelength channel optical power is provided. The diagram provides oneexample of the data within the optical signal which is being transmittedbetween nodes 102 a and 102 b. Within the high-speed data 200 is a pilottone 202, which is a small and low frequency (tens of MHz or below)modulation, applied to a high speed (many Gbps) wavelength channel.Typically, each wavelength channel 104 is assigned its own unique lowfrequency of modulation; thus, a low-frequency electrical spectrum of asmall portion of the high-speed data 200 detected by the pilot tonedetector 106 is representative of the wavelength channels present.Furthermore, the low-frequency signal can itself be modulated withchannel-specific information such as modulation format, modulation rate,etc., enabling the pilot tone to be used for supervisory, control,equalization, continuity, synchronization, or reference purposes for theoptical telecommunications system 100. In the current disclosure, thepilot tone is generated in the digital domain prior to being convertedinto an analog signal and transmitted together with the high speed datasignal. A pilot tone detector 106 at the destination node (or at anyplace within the optical telecommunication system 100) is then able tomonitor channel information associated with all of the wavelengthchannels 104 within the optical system 100 as information is beingtransmitted between nodes 102.

Turning to FIG. 3a , a schematic diagram of a transmitter within one ofthe nodes 102 a-102 g is shown. The transmitter, such as the onementioned in FIG. 1b , 112 includes an occurrence modulation portion 302which is connected to an electrical-to-optical converter (E/O) 304. Inone embodiment, the occurrence modulation portion 302 is located withina digital signal processor (DSP) or a transmitter that is part of anoptical communications link.

The occurrence modulation portion 302 includes a data driver orprocessing portion 306 that provide digital data having a data rate andan optional pre-conditioning apparatus 308 for pre-conditioning thedigital data to include an amount of randomness to the digital data. Fordigital data already including a degree of randomness due to priorprocessing, imperfect electronics, interference, etc., thepre-conditioning apparatus 308 may not be required. The occurrencemodulation portion 302 further includes a modulator, such as multiplier310, for modulating the digital data and a digital-to-analog converter(DAC) 312. In one embodiment, the DAC is a low resolution DAC. Due tohigh operational speeds, a DAC usually has a limited number of bits withan effective number of bits (ENOB) of less than 6 bits although thisnumber may be different for different DACs. Depending on the system, themodulator may be an amplitude, frequency or phase modulator.

In one embodiment, the transmitter 112 is a coherent transmitter havinga dual polarization I/Q modulator. In the coherent transmitterembodiment, there are four independent data transmission streams.Therefore it should be understood that the node 102 includes fourparallel occurrence modulation blocks or portions 302 for thatembodiment.

In one embodiment, structurally within the occurrence modulation portion302, the data driver 306 is connected to the pre-conditioning apparatus308 which has its output connected to a first input 313 of themultiplier 310. A second input 314 to the multiplier 310 introduces amodulation factor to the first input 313. In one embodiment, the firstinput 313 is modulated with the factor of 1+d(t)*m_(d) sin(2πf_(m)t+φ)where d(t) is the pilot tone data (its value can be 1,0, or −1) to betransmitted, m_(d) represents a modulation depth, f_(m) represents afrequency value and φ represents a phase value. An output of themultiplier 310 is connected to an input of the DAC 312 which, in turn,is connected to the E/O 314. In another embodiment, modulation of theamplitude and/or the phase of the data can also be performed. In thisembodiment, a modulation factor represented by equationm(t)=A(t)exp(jφ(t)) may be used as the second input 314, where A(t) isthe amplitude and φ(t) is the phase modulation.

Turning to FIG. 3b , a flowchart of a method for generating a pilot tonefor an optical telecommunications system is shown. After it isdetermined that data is to be sent from a transmission node 102 a to adestination node 102 b, the data is generated and sent to thetransmitter 112 for transmission to the destination node 102 b. Thedata, or digital data, preferably includes a data rate. After receivingthe data in the digital domain, the data that is to be transmitted ispushed or driven (1300) by the data driver 306 to the pre-conditioningapparatus 308. The digital data may then be pre-conditioned (1302) bythe pre-conditioning apparatus 308 to add an amount of randomness to thedigital data. In one embodiment, pre-conditioning can be achieved byadding a small random value (no greater than +/−0.5 of the leastsignificant bit (LSB) of the DAC) to each original data point. Thepre-conditioning may not be required when the signal already carriessome randomness component to it due to noise, imperfect circuitry, etc.,or naturally achieved when other functionalities are performed, such as,but not limited to, pulse shaping. After being pre-conditioned, theoutput of the pre-conditioning apparatus 308 is then modulated (1304) bymultiplying the output of the pre-conditioning apparatus with amodulation factor to produce a modulated digital output. This processmay be seen as occurrence modulation. The occurrence modulation assistsin generating a pilot tone, in the digital domain, for the data beingtransmitted. Typically, the modulation of the digital data is performedat a pilot tone frequency lower than the data rate. In one embodiment,the modulation factor is 1+d(t)*m_(d) sin(2πf_(m)t+φ) where d(t) is thepilot tone data to be transmitted, m_(d) represents a modulation depth,f_(m) represents a frequency value and φ represents a phase value. Afterbeing modulated, the modulated digital output is then transmitted to theDAC 312 which converts (1306) the modulated digital output from a signalin the digital domain to an analog signal for transmission to thedestination node 102 b over communication channels within the opticaltelecommunications system 100. In one embodiment, the modulation in 1304may be preferably performed to a peak-to-peak modulation depth less thana LSB of the digital-to-analog conversion and more specifically, may beperformed to a peak-to-peak modulation depth less than 10% of thedigital-to-analog conversion.

The analog signal is then converted (1308) from an electric signal to anoptical signal by the E/O 314 before being transmitted (1310) to thedestination node.

Turning to FIGS. 4a to 4c , sample charts illustrating varioussimulations using the transmitter of FIG. 3 are shown. Using data of aGaussian random number with a sigma of 1 and a mean of 0 and a DAC 312in the range of −3 to +3, it is shown that a minimum achievablemodulation depth is not limited by the resolution of the DAC 312. In thecurrent simulation, the data is modulated (at the multiplier 310) by themodulation factor 1+0.01 sin(2πf_(m)t) before being transmitted to theDAC 312 where the modulation depth is 0.01 and the pilot tone frequencyis f_(m)=25 MHz.

While it may be assumed that the minimum modulation depth would bedetermined by the formula 2/2^(n) for each (n)bit DAC, the system of thedisclosure allows for more control of the modulation depth.

As shown in FIGS. 4a to 4c , on the Y-axis spectral power is shown inarbitrary units (a.u.) with frequency (in MHz) on the X-axis. FIG. 4a isdirected at a simulation using a 6-bit DAC, FIG. 4b is directed at asimulation using a 5-bit DAC while FIG. 4c is a simulation using a 4-bitDAC. For each of the simulations, at 25 MHZ, the spectral power wasaround 1×10⁸ arbitrary units, which represents 2% peak to peak amplitudeof the signal being modulated. It is to be noted that the 4-bit DAC hasa resolution limit of 1/2⁴=6.25%. Therefore, the graphs of FIGS. 4a to4c indicate that a pilot tone of an amplitude less than a resolutionlimit of a DAC can be readily generated.

Turning to FIGS. 5a to 5d , graphs illustrating an intensity modulationof an optical signal are provided, assuming the original data withoutmodulation has a Gaussian distribution of an optical power level. FIGS.5a to 5d represent results of different modulation depths using a same6-bit DAC having a resolution limit of 1/2⁶, which is approximately1.6%. The optical signal power is generally proportional to the squaredoutput voltage of the DAC 312. In equation form, the power may be seenas |V_(m,DAC)(t)|²−|V_(0,DAC)(t)|². In the graphs, the Y-axis and X-axisrepresent the optical intensity difference with and without modulationand the sampling point index, respectively.

In FIG. 5a , a graph showing results for an optical signal having amodulation depth of 30%, while FIG. 5b reflects results for an opticalsignal having a modulation depth of 10%. The graph of FIG. 5c reflectsresults for an optical signal having a modulation depth of 5%, whileFIG. 5d reflects the output of an optical signal having a modulationdepth of 1%.

Turning to FIGS. 6a to 6d , various graphs showing optical signal powerlevel having an identical modulation depth but for different DACs isshown. FIG. 6a shows the optical signal power for an optical signalhaving a modulation depth of 1% for a 6-bit DAC. This is identical tothe FIG. 5d graph discussed above. FIG. 6b is a graph showing theoptical signal power with a modulation depth of 1% for a 5-bit DAC.FIGS. 6c and 6d are graphs showing an optical signal power for amodulation depth of 1% for a 4-bit DAC and a 3-bit DAC, respectively. Ascan be seen, the modulation occurrence decreases as smaller bit DACs areused.

FIGS. 7a and 7b are graphs showing results when the applied modulationis smaller than the DAC LSB. In FIG. 7a , the original data is constant,and the applied modulation is smaller than 1 LSB. In this graph, nothingis changed to the DAC output as if no modulation is applied. In FIG. 7b, due to randomness in the data, the DAC output may be occasionallychanged by 1 LSB, leading to occurrence modulation.

Turning to FIGS. 8a and 8b , graphs reflecting a pilot tone generated byoccurrence modulation and pilot tone power fluctuation dependence on thenumber of DAC bits are provided. In these figures, the pilot tone hasthe same frequency as the amplitude modulated signal (FIG. 8a ), howeverthe fluctuation of the pilot tone power decreases as the quantizationresolution increases (FIG. 8b ). As is understood, the quantizationresolution is based on the number of bits in the DAC being used.

The graph of FIG. 8a represents a 6-bit DAC (with spectral power on theY-Axis and frequency (in MHz) on the X-Axis) when a 2%, 25 MHz pilottone is applied. For the graph of FIG. 8b , the Y-Axis representsrelative tone power and the X-axis represents the different simulationsthat were performed (numbered 1 to 10) for different bit DACs asidentified in the legend. The line including a hollow circle representsthe results for a 2-bit DAC, the line with the hollow square representsthe results for a 4-bit DAC, the line with the hollow trianglerepresents a 6-bit DAC while the line with the filled circle representsan 8-bit DAC. As is seen, a lower DAC resolution leads to increasedfluctuation in the pilot tone power since a lower DAC resolution hasless occurrence modulation.

Turning to FIG. 9, a graph reflecting a normalized standard variation ofpilot tone power vs. number of bits is shown. On the Y-axis is thenormalized standard deviation of the pilot tone power and on the X-axisis the number of bits. In the graph, N=224000 points are simulated, themodulation depth is 0.01 and the V_(max) is 3. As understood instatistics, for a random number with mean X, its standard deviation is1/sqrt(X). In order to calculate the pilot tone variation, number ofoccurrence, N_(eff), is to be calculated. If V_(LSB) is the voltage of 1LSB, m_(d) is the modulation depth, then the probability of changing 1LSB is m_(d)/(πV_(LSB)/2). For N points, the number of points changed bythe modulation is N_(eff)=m_(d)/(πV_(LSB)/2). Therefore the standarddeviation is given by

$\sigma = {\frac{1}{\sqrt{N_{eff}}} = {\frac{1}{\sqrt{N\frac{2}{\pi}\frac{m_{d}}{V_{LSB}}}} = \frac{1}{\sqrt{N\frac{2}{\pi}\frac{m_{d}}{V_{{ma}\; x}/2^{{nbits} - 1}}}}}}$

FIGS. 10a and 10b are graphs reflecting experimental verificationshowing power (in dB) as a function of frequency. In the graphs, theY-axis represents power (in dBm) the X-axis represents Frequency (inkHz). The testing was performed with a PM-QPSK Transmitter with a 6-bitDAC having a buffer length of 2¹⁷. The DAC sampling rate is 56 GHz. Theminimum frequency space was 28 GHz/2¹⁷ or 213.623 kHz. For demonstrationpurposes, the peak-to-peak modulation was approximately 2%. Differentmodulations frequencies were then applied to X and Y polarizations.

In the experiment, the applied pilot tone frequency for X polarizationwas 56 GHz/2064 or 27.13178 MHz and the applied pilot tone frequency forY polarization was 56 GHz/2048 or 27.34375 MHz. The 3dB spectral widthis less than 2 kHz and is only limited by the measurement bandwidth. Thegraph shown in FIG. 10a reflects the power measured between thefrequencies 27.1 and 27.5 MHz while the graph shown in FIG. 10b showsthe power measured between 27.338 and 27.35 MHz frequencies.

One advantage of the current disclosure is that the implementation ofthe system for creating a pilot tone is simplified, since an externalvariable optical attenuator or an optical modulator, per each wavelengthchannel, is not required. This can represent significant cost savingsfor a wavelength division multiplexed (WDM) system utilizing 80-100wavelength channels per optical fiber. By using digital apparatusalready present within the node 102 a-102 g, such as the transmitter112, a digital pilot tone can be added to data to be transmitted betweennodes 102. By including occurrence modulation in the generation of thepilot tone, a modulation depth of less than 1 LSB may be realized. Thisdigital pilot tone can then be converted to an analog equivalent alongwith the data to produce the optical signal to be transmitted. Anotheradvantage of the current disclosure is that there is flexibility for thecreation of the pilot tone. In other words, any single frequency (fromkHz to GHz) can be generated. A spectrum spreading modulation may alsobe generated.

A further advantage that is realized by the system of the disclosure isthat since the pilot tone is added in the digital domain, the modulationdepth can be controlled to be more accurate and there is little or noneed for calibration or feedback control. This results in improved powermonitoring accuracy over some current solutions and an easierimplementation. Furthermore, other forms of modulation can be realizedsuch as phase or frequency.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms within departing from the scope ofthe present disclosure. The present examples are to be considered asillustrative and not restrictive, and the intention is not to be limitedto the details given herein. For example, the various elements orcomponents may be combined or integrated in another system or certainfeatures may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the inventive concept(s)disclosed herein.

What is claimed is:
 1. A method of generating a pilot tone for anoptical telecommunications system, the method comprising: providingdigital data having a data rate, for transmission to a destination node;modulating the digital data at a pilot tone frequency lower than thedata rate, to provide a modulated digital output; and converting themodulated digital output to an analog signal to generate the pilot tone.2. The method of claim 1, wherein modulating the digital data isperformed to a peak-to-peak modulation depth less than a leastsignificant bit (LSB) of the digital-to-analog conversion.
 3. The methodof claim 2, wherein the peak-to-peak modulation depth is less than 10%of the LSB of the digital-to-analog conversion.
 4. The method of claim2, further comprising, before modulating the digital data,pre-conditioning the digital data to add an amount of randomness to thedigital data.
 5. The method of claim 4 wherein pre-conditioning thedigital data comprises adding a random or pseudo-random value of nogreater than 0.5 of the LSB to each data point within the digital data.6. The method of claim 1 wherein modulating the digital data comprisesmultiplying the digital data by a modulation factor, wherein themodulation factor equals 1+m_(d) sin(2πf_(m)t+φ) where m_(d) representsa modulation depth, f_(m) represents a frequency value and φ representsa phase value.
 7. The method of claim 1 wherein modulating the digitaldata comprises: modulating an amplitude, a phase, or a frequency of thedigital data.
 8. The method of claim 1 further comprising: convertingthe analog signal to an optical signal for transmission to thedestination node.
 9. A system for generating a pilot tone for an opticaltelecommunications link, the system comprising: a data driver forproviding digital data having a data rate; a modulator for modulatingthe digital data provided by the data driver at a pilot tone frequencylower than the data rate, to provide a modulated digital output; and adigital-to-analog converter (DAC) for receiving the modulated digitaloutput and converting the modulated digital output to an analog signalto generate the pilot tone.
 10. The system of claim 9, wherein themodulator is configured to modulate the digital data at a peak-to-peakmodulation depth less than a least significant bit (LSB) of the DAC. 11.The system of claim 10, wherein the peak-to-peak modulation depth isless than 10% of the LSB of the DAC.
 12. The system of claim 10, furthercomprising a pre-conditioning apparatus configured for pre-conditioningthe digital data to add an amount of randomness to the digital data. 13.The system of claim 12, wherein the pre-conditioning apparatus isconfigured to add a random or pseudo-random value of no greater than 0.5of the LSB to each data point within the digital data.
 14. The system ofclaim 9 wherein the modulator comprises: a multiplier for multiplyingthe digital data by a modulation factor.
 15. The system of claim 14,wherein the modulation factor equals 1+m_(d) sin(2πf_(m)t+φ) where m_(d)represents a modulation depth, f_(m) represents a frequency value and φrepresents a phase value.
 16. The system of claim 9, wherein the DAC isa low-resolution DAC.
 17. The system of claim 9, wherein the system islocated within a digital signal processor (DSP).
 18. The system of claim9, wherein the system is located within a transmitter of the opticalcommunications link.
 19. The system of claim 9, wherein the modulatorcomprises an amplitude, a phase, or a frequency modulator.
 20. Thesystem of claim 9, further comprising an electro-optical converter forconverting the analog signal to an optical signal for transmission inthe optical telecommunications link.